Dynamometers are widely used for testing motor vehicles in place to permit fuel consumption, exhaust gas pollutants etc. to be measured. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertia (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle. In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=A+BV+CV.sup.2 +DW
where A represents the vehicle constant load coefficient (e.g., effects of breakaway force), and, B and C represents the vehicle load coefficient dependent on velocity and velocity squared (e.g., windage), and D represents the grade coefficient (e.g., slope of the grade). It should be noted that a load coefficient based on velocity cubed may be added if desired). V represents the vehicle velocity and W represents the vehicle weight.
The purpose of the dynamometer is to impose those forces on the vehicle which the vehicle would incur during actual operation on a road. Such dynamometers include a roll (or a pair of rolls) for engaging the driven wheel (e.g., motorcycle) or wheels (e.g., automobile, truck, bus etc.) of the vehicle being tested. It should be noted that where 4 or 6 wheel drive vehicles are to be tested the dynamometers will include 2 or 3 pairs of rolls. The roll or rolls are generally supported by a shaft at each end of the roll which is journaled in a bearing mounted on a frame.
Typically a power absorbing and supplying unit such as an electric motor or a power absorber per se such as an eddy current brake, friction brake or hydrokinetic brake is coupled to the roll for absorbing power from the roll. The roll in turn applies a retarding force to the surface of the vehicle wheel (e.g., tire) to simulate the road load and inertial forces. Inertial forces can be simulated by an electric motor during acceleration as well as deceleration but can only be simulated by a power absorber/brake during acceleration because such braking units only absorb and do not supply power. Generally (and often when electric motors are used) a large part of the inertial forces are simulated by selectively coupling the roll to one or more mechanical flywheels. The combined inertia of the flywheels, the roll and other rotating components exert a force on the vehicle wheels proportional to the acceleration (or deceleration) of the vehicle wheels. Thus, the engine is required to expend as much power in accelerating the roll as it does in overcoming the vehicle inertia during actual road acceleration. Road load and inertial forces which must be simulated by the dynamometer may be calculated from the formula: EQU F=A+BV+CV.sup.2 +DW+Idv/dt
wherein I represents the simulated inertia, dv/dt the derivative of velocity with respect to time (or the acceleration or deceleration of the vehicle).
While electric motors (power supplying and absorbing units) increase the versatility of a dynamometer they also significantly increase the cost. For that reason, dynamometers equipped with power absorbers which provide braking only are in considerable demand for mass vehicle emission testing stations, mandated by the U.S. Environmental Protection Agency ("EPA"), as well as for facilities which must provide the necessary repairs so that rejected vehicles can pass a renewed emissions test. Both friction brake and eddy current brake dynamometers are typically less expensive to manufacture than dynamometers equipped with electric motor or hydrokinetic absorbers. However, friction brakes are subject to wear and thus require considerable maintenance. Eddy current brake dynamometers do not suffer the wear problems associated with friction brake machines and are particularly cost effective for EPA mass-in-service testing for vehicle emissions and repair applications.
Dynamometers utilizing an eddy current brake (or other power absorbing units) can be designed with a base or minimum inertia i.e., the smallest vehicle inertia which can be simulated and a maximum inertia, i.e., the largest inertia which can be simulated. The minimum dynamometer inertia is made up of the inertia of the roll or roll set, the inertia of the rotating components of the absorber and a trim inertia. The term roll or roll set inertia, as used herein, shall mean the rotational inertia of the roll or roll set which engages the vehicle wheel(s) plus the inertia of minor inertia contributing auxiliary components such as the shaft(s), couplings and gears or belts, where used, which are permanently connected to the roll. The inertia of such auxiliary components is less than that of the roll or roll set.
The trim inertia in a typical dynamometer is that inertia required to bring the total inertia of the parts of the dynamometer permanently connected to the rolls up to the designed minimum inertia. The trim inertia in a dynamometer utilizing an eddy current brake (or other power absorbing unit) is typically made up of a fixed flywheel, i.e., a mechanical flywheel permanently connected to the roll. As an example, an eddy current brake dynamometer designed to test motor vehicles weighing 2000 pounds or more would include a roll set inertia of say 300 pounds, a braking unit inertia (i.e., the rotor inertia) of say 200 pounds and a fixed flywheel inertia of 1500 pounds. Additional flywheels, clutched to the roll shaft, generally make up the difference between the dynamometer's minimum inertia (e.g., 2000 pounds) and its maximum inertia (e.g., 5000 to 6000 pounds). It should be noted that the braking unit per se may be used to simulate the difference between the base and maximum inertia during acceleration only, as pointed out above.
Eddy current brakes comprise a rotating wheel made of a suitable electrically and magnetically conducting material with sufficient strength to withstand the centrifugal and other forces encountered. Stationary field coils are positioned adjacent the rotor for generating an exciting magnetic field which in turn induces eddy currents in the rotating rotor. Eddy currents generate the secondary high density magnetic field in a gap between rotor and coils which causes the absorber to apply a braking torque to the roll and to the vehicle tire in contact with the roll. The amount of braking torque is determined by the density of the magnetic field which is controlled by the current supplied to the field coils.
Typical eddy current brakes are designed to minimize the rotor mass so that energy is not wasted in accelerating or decelerating extra mass.
In our prior applications we pointed out that contrary to the prevailing view, there are advantages to increasing the rotor mass of an eddy current brake so that the brake rotor makes up the dynamometer's trim inertia. The advantages include the elimination of the need for a separate trim inertia flywheel (and bearings) and the accommodation of higher power absorptions. Also, the large rotor mass results in a lower magnetic field strength, lower power consumption and lower magnetic saturation of the rotor iron as compared with a conventional eddy current brake having a comparable short term braking capacity. In our prior applications the eddy current brake was rotatably mounted on the dynamometer frame and connected to the roll which engages the driven wheel of the vehicle through a suitable coupling. We have since discovered that the inclusion of the brake rotor as an integral part of the roll provides additional advantages in terms of reducing the material costs, labor cost and the overall length of the dynamometer.